In this chapter Laurie Spiegel, a pioneer of algorithmic logic in music composition, considers various reasons to use algorithms, including their function as descriptors, generators and adjuncts to creative musical practises. Self-simulation (notably, of decision making processes) is juxtaposed against the sonification of external information and various other uses of algorithms are also described. Human input may be minimal or extensive for the logic used to specify parameters of individual sonic events, variations in global informational entropy, inherent structuring or to achieve variation of material. Spiegel values algorithms particularly to allow her to ‘inhabit the state of flow’ of music by freeing her to focus on selected aspects of composing while handing off other aspects to automated procedures. The chapter includes descriptions of the kinds of uses of algorithmic logic that have contributed to the composition of specific musical works.